Fluorine-containing elastomers comprising vinylidene fluoride-tetrafluoroethylene-perfluoro(methyl vinyl ether) as the main structural unit have not only distinguished heat resistance and solvent resistance peculiar to the fluorine-containing elastomer, but also have good low-temperature characteristics, and thus have been so far used in many industrial fields including the automobile industry. However, even the fluorine-containing elastomers often fails to satisfy the recent technical progress, where particularly more strict requirements for the low-temperature characteristics and resistance to alcoholic solvents such as methanol, etc. are imposed. Due to the recent exhaust gas regulations, etc., further requirements for the heat resistance, solvent resistance, and low-temperature characteristics have been also imposed on the fluorine-containing elastomers.
To solve the problems, it has been so far proposed to prepare the fluorine-containing elastomer by copolymerizing a monomer having a plurality of ether bonds on the side chains in place of perfluoro(methyl vinyl ether), where the monomer must be copolymerized in a high proportion to obtain copolymers in an elastomeric state, whereas in a low proportion only semi-resinous state copolymers will be obtained, resulting in deterioration of the low-temperature characteristics. Actually, 12-50% by mole is recommended for the copolymerization proportion of the monomer, and all of Examples in the following reference teach copolymerization proportions of 25-32% by mole. However, such fluorine-containing elastomer containing the monomer in a high proportion shows a poor mechanical strength and moreover has such a problem as a poor molding processability, e.g. easy occurrence of foaming at the time of molding.
Patent Literature 1: JP-B-5-13961
Perfect fuel oil resistance is required for automobile fuel seal materials, and thus commercially available fluororubber is now used mainly for this purpose. Besides the ordinary gasoline, oxygen-containing fuels such as ether, alcohol, etc. have been now used as automobile fuels from the viewpoint of combustion efficiency, etc. Fluororubber having an increased fluorine content can respond to the oxygen-containing fuel, but the increased fluorine content can deteriorate the cold resistance, giving rise to fear of fuel leakage in winter cold districts, whereas a decreased fluorine content can improve the cold resistance, but can lose the resistance to the oxygen-containing fuel. It is now very hard to satisfy these two requirements at the same time.